This invention relates to an automatic masking device including an aperture extension means, used in a printer for plate-making, or the like, in which an original picture is projected, through a focusing lens, onto a photosensitive material such as a film, a plate, or the like, mounted to a photosensitive material holder.
When a plurality of original pictures are printed one by one onto a photosensitive film according to the desired layout by using a grouping camera or a projector, each original picture is projected onto the film through a focusing lens and an aperture of a masking device which masks the surface of the photosensitive film except the corresponding portion of each picture.
In a conventional method, when the edge portion c of an original picture b having a dark opaque frame a, as shown in FIG. 1, projected onto a focusing glass is masked by using a masking device, the dark frame a of the picture b is prone to be confused with an inner edge e of a mask, with a result of a wrong trimming.
Further, it is rather difficult and troublesome to carry out a trimming of a part of a small size with respect to the original picture through the aperture of the mask.